The present invention relates to path test apparatus and a method for use in a telephone system operating in time frames each consisting of a plurality of time slots.
Pulse code modulated (PCM) techniques used with telephone switching enables a multiplicity of conversations to be transmitted over a two wire digitally multiplexed line commonly known as a T1 trunk line. In a telephone system operating with a PCM format, PCM signals on T1 trunk lines are multiplexed together into what is known as a line group. A line group will connect serial PCM data to parallel format of 8 bits of data. Multiplexing up to 16 T1 lines each having 24 channels of PCM data thereon will provide 384 time slots per line group to be switched through the system. In the above identified application, entitled "Integrated Message Accounting System", up to 4 line groups may be accommodated, thereby providing 1536 time slots. The system is capable of switching PCM data from one time slot in one line group to any other time slot in any other line group.
There is a need for a path test scheme to insure that a particular path is in fact set up through the digital switch. Otherwise, a subscriber in a tributary office initiating a 1+ DDD call could be switched to an improper time slot in another line group thereby preventing proper completion of his call to the called party.
Because the electronic digital design of a PCM telephone system is capable of extremely high switching speeds, the path test scheme must be capable of being performed in an extremely short time period in order for the system to make proper connections from one channel time slot to another.
Prior art systems, in addition to the 8 bits of data, add a parity bit (9 bits total for each time slot), and another bit which is a test bit and is transmitted along with the PCM data bits. Prior art systems thus require a tenth bit in order to test the continuity of a particular path set up between locations in a telephone system.
In accordance with the above background, it is desirable to provide path test apparatus and a method that does not require the use of additional hardware, such as required for a tenth bit, to test the continuity of a data path in a telephone system.